Visualizing Seasonal Data in R

The end of the year is a very important time for retailers. A quick look at some data from the CSO database shows why. It's clear in the figure below that there is a significant spike in retail sales every December.

Now let's imagine that you want to analyse this time series in R, but rather than looking at the whole series, you want to focus on the data for the month of December. In that case, you might like to make a plot similar to the one I've created below.

To be able to create a graph like the one above you need to extract the data points for each December from the whole series. There are probably many ways of doing it, but I've written a short R script that demonstrates just two possible methods. The first method is a bit hackish and the second method is a bit more systematic. The script also shows how I labelled the x-axis.

Note that the basic idea behind the first method is to extract every nth element of a vector in R while the second method involves sub-setting a ts object.